Some 14 other guard troops were wounded in the blast in the city of Khoramabad, some 300 miles south-west of the capital, Tehran.

The country's state news agency said the blast was caused by a fire that had reached the ammunition storage area, but there was no word on what had ignited the blaze.

In their first reports of the blast late on Tuesday, most Iranian news agencies said the explosion was an accident.

Although Khoramabad has not seen violence recently, it is geographically close to Kurdish-populated areas that have seen several attacks in recent months by Kurds disgruntled with the central government.

Iran is battling armed militant and separatist movements in the remote south-east along the border with Pakistan and in the far north-west along the border with Iraq.

On September 12, a blast at a military parade in Mahabad, near Iraq, killed 12 people and prompted a cross-border retaliatory raid by Iranian forces. They blamed the attack on Kurdish separatists and followers of former Iraqi dictator Saddam Hussein.